In planar integrated photonics, spatially periodic structures are very useful for manipulating optical propagation. Often, this requires high-aspect-ratio features defined by surfaces perpendicular to a substrate. For example, semiconductor laser diodes, especially in-plane (or edge-emitting) laser diodes, are key components of many optical/optoelectronic integrated circuits. Currently, the material- and structure-dependent lasing wavelength extends from 300 nm to longer than 10 μm and emission power greater than 1 kW for high power and 1 W for coupling to single mode optical fibers are available from compound semiconductor technology. This lasing performance is achieved using a Fabry-Perot optical resonator that is bounded by cavity mirrors perpendicular to the gain length. Typically these are perpendicular to the substrate surface for in-plane laser diodes.
Semiconductor lasers require high optical quality mirrors for good device performance. Conventional methods for making these mirrors require cleaving of a substrate. In a (001) oriented semiconductor substrate, cleaving is along a <110> direction and provides reflectivity of about 0.3-0.4 without additional coatings. However, this is not compatible with integration processes because it involves separating a large wafer into many small pieces, each of which must be carefully handled and integrated with other components.
Another conventional method for mirror fabrication for integrated circuits is chemical etching using either wet or dry etching processes. But an etched surface is generally not perfectly vertical to the substrate and is not atomically flat. This results in light scattering losses that are detrimental to the lasing process. For integration of laser diodes into a large scale optical/optoelectronic integrated circuit, a process that can overcome these problems is required.
Thus, there is a need to overcome these and other problems of the prior art and to provide a method to fabricate mirror facets on a substrates of Group III-V compounds and/or Group IV semiconductors compatible with photonic integration processes.